1. Field of the Invention
The present invention relates to a solid state imaging device having a high quality picture and low power consumption, and a method of driving the same.
2. Description of Related Art
Solid state imaging devices carried in cellular phones or the like include a CCD (charge coupled device) type image sensor and a CMOS type image sensor. The CCD type image sensor is excellent in picture quality, and the CMOS type image sensor consumes lower power and its process cost is low. In recent years, MOS type solid state imaging devices using a threshold voltage modulation method which combines both high quality picture and low power consumption have been proposed. The MOS type solid state imaging device using the threshold voltage modulation method is disclosed in Japanese Unexamined Patent Publication No. 2002-134729, for example.
In the solid state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, picture output is obtained by arranging unit pixels in a matrix form and repeating three states of initialization, accumulation, and reading. Moreover, in the solid state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, each unit pixel has a photo-diode, a modulation transistor, and an overflow drain gate. The gate of the modulation transistor is formed in a ring shape.
Electric charges (photo-generated electric charges) generated by light incident upon the photo-diode are transferred to a P-type well region formed under a ring gate, and accumulated in a carrier pocket formed in this region. The threshold voltage of the modulation transistor changes corresponding to the photo-generated electric charges accumulated in the carrier pocket. Accordingly, signal (pixel signal) corresponding to incident light is obtained from a terminal coupled to the source region of the modulation transistor.
In the solid state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, in order to prevent distortion produced when imaging a moving object, a picture made by an optical signal is simultaneously taken in on an entire light-receiving surface, and the optical signal thereof is converted to an electric signal and taken out to the outside as a picture signal. However, in this reading method, an accumulating period and a reading period cannot be controlled for each pixel because the picture made by the optical signal is simultaneously taken in on an entire light-receiving surface. Therefore, while specific pixels are being read, the accumulation operation cannot be carried out in other pixels, and as a result the frame rate cannot be enhanced.
Moreover, in the solid state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, the photo-generated electric charges accumulated in a carrier pocket is discharged to an area under a P type well through an N type layer at the time of initialization. In other words, the photo-generated electric charges are discharged to a substrate through the N type layer. Therefore, the P type well having the carrier pocket is required to store a carrier during a reading period, and required to discharge this carrier during a discharging period. In order to satisfy such contradicting requirements, the thickness and an impurity concentration of each impurity layer should be precisely controlled, and the design flexibility will be remarkably damaged.
Furthermore, in the solid state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, a part of the photo-generated electric charges generated in the photo-diode are discharged to the substrate through an overflow-drain region formed in a P type layer.
Incidentally, in the N type layer which constitutes the photo-diode, as the peak of the impurity concentration becomes deeper perpendicularly downward in the substrate, the wavelength range of photoelectric-convertible incident light becomes wider. That is, taking a higher quality picture into consideration, the depth of the N type layer needs to be deep enough. On the other hand, the overflow-drain region is composed of a deep P type layer extending from underneath an overflow drain gate to the rear surface of the substrate. This P type layer is formed by implanting P type impurities after forming the above-described N type layer. Therefore, in order to form the overflow-drain region composed of a deep P type layer, an ion-implanting energy needs to be increased. Generally, the area of an impurity region formed by increased ion-implanting energy increases. That is, the area of the overflow-drain region will increase. Therefore, there is a tradeoff relationship between a higher quality picture and miniaturization, and there is also a problem in that a solid state imaging device cannot be miniaturized if the N layer is deepened for achieving higher quality picture.
The present invention has been made in view of the above described problems, and is intended to provide a solid state imaging device and a method of driving the same which can attain, without damaging the design flexibility, a higher quality picture and miniaturization while speeding up the frame rate by allowing the accumulating period and the reading period to be set to a common timing.